The present invention relates to a conversion catalyst for ethylbenzene containing xylenes and a conversion process by using the catalyst, more particularly, a process in which ethylbenzene containing xylenes are made to contact with a specific catalyst in the presence of hydrogen, thus effecting dealkylation of ethylbenzene mainly into benzene and at the same time attaining isomerization of ortho-xylene and/or meta-xylene into para-xylene.
Of xylene mixtures, para-xylene is now in particular an industrially important product. The demand of para-xylene has remarkable increase as a crude raw material for polyester, a synthetic fiber. It is expected that para-xylene will continue to have such increase in the demand. Ortho-xylene and meta-xylene which are xylene isomers other than para-xylene are extremely lower in the demand than para-xylene, thus making it industrially important to convert them into para-xylene.
Since individual isomers of xylene and ethylbenzene are close in their boiling point, it is practically impossible to separate para-xylene through distillation method. Thus, low-temperature separation and adsorption separation processes are used for this purpose. The low-temperature separation process is restricted in recovery of para-xylene for the para-xylene recovery rate per one pass due to eutectic point, thus resulting in relatively high concentrations of para-xylene in raffinate fluid after recovery of para-xylene. In the low-temperature separation process the recovery rate of para-xylene per one pass can be improved with the high concentration of para-xylene contained in the supplied raw material.
In contrast, the adsorption separation process is able to recover para-xylene at 100% for one pass. Namely, the concentration of para-xylene in raffinate fluid after the adsorption and separation is extremely low or it can be reduced to almost zero. However, in this process, it is ethylbenzene that inhibits most the separation of para-xylene among C8 aromatic hydrocarbon mixtures. Thus, the reduced concentration of ethylbenzene in a raw material supplied for adsorption and separation makes it possible to improve the adsorption and separation function of para-xylene and increase the concentration of para-xylene in the raw material supplied, thereby improving the capacity of producing para-xylene in the same facility for adsorption and separation.
Therefore, raw materials for xylene to be supplied in the separation process should be those in which the concentration of ethylbenzene is kept as low as possible and the concentration of para-xylene in xylene kept as high as possible by which the concentration of para-xylene can be kept high in C8 aromatic hydrocarbon mixtures.
In general, industrially available xylene raw materials are reformed xylenes which are obtained through reformation of naphtha and subsequent aromatic extraction and/or fraction or cracked xylenes which are obtained by subjecting decomposite gasoline (by-product of thermal decomposition of naphtha) to aromatic extraction and/or fraction. Cracked xylenes are characterized by two-times higher concentration of ethylbenzene than that of reformed xylenes, a representative ingredient of which is shown in the table below.
TABLEIngredients of xyleneIngredientsreformedcrackedEthylbenzene18 weight %39 weight %Para-xylene1913Meta-xylene4232Ortho-xylene2116
As shown in the above, in general, xylene mixtures have a substantial quantity of ethylbenzene. Failure in removing ethylbenzene by any means would result in an undesirable situation where ethylbenzene accumulates after repetition of separation and isomerization steps, thus resulting in a higher concentration of ethylbenzene. Under these circumstances, reformed xylenes lower in the concentration of ethylbenzene are now used as a preferable source as freshly supplied raw materials. However, in recent years when a limited availability of petroleum has caught attention, reevaluation is made for thermally cracked xylenes as another xylene source. In any case, it is necessary to reduce the concentration of ethylbenzene, for which several processes have been proposed and some of them have been actually done on an industrial scale. These processes can be roughly classified into a process in which ethylbenzene is separated as it is and another process in which ethylbenzene is converted into other useful compounds through reactions.
Distillation is a process for separation of ethylbenzene. In this process, ultra-precision distillation is needed due to a small difference between the boiling point of ethylbenzene and that of xylene, thus requiring a great amount of investment for commercial production facilities and making the operational cost higher and economically unfavorable. There is another presented process by which adsorption and separation process is employed to separate ethylbenzene. This process is, however, not well satisfactory in the separation function.
Other processes for removing ethylbenzene include those for converting ethylbenzene into useful ingredients. The representative processes are shown below:
(1) a process for converting ethylbenzene into xylene (for example, refer to Japanese Patent No. 1974-46606 (the embodiment 3 on page 3),
(2) a process for converting ethylbenzene into benzene and diethylbenzene through disproportionate reaction (for example, refer to Japanese Patent No. 1978-41657 (on 32nd–33rd line, 19th column, page 10) and
(3) a process for converting ethylbenzene into benzene and ethane through dealkylation reaction (for example, refer to Japanese Patent Laid-Open No. 1982-200319 (embodiments from 2 to 4 on pages from 7 to 8).
Of the above process, the process (1) for converting ethylbenzene into xylene indispensably needs platinum, a quite expensive precious metal, to be contained in a catalyst. Further, conversion of ethylbenzene into xylene needs the presence of non-aromatic ingredients such as naphthene and paraffin in view of reaction mechanism, with the concentrations of such ingredients in the obtained product ranging from several to 10+several percentages. The process is also controlled for conversion of ethylbenzene by thermodynamic equilibrium and therefore restricted thereby. These matters are disadvantages of this process.
The above process (2) is for converting ethylbenzene into benzene and diethylbenzene through the disproportionate reaction. Benzene produced by the process is then hydrogenated to cyclohexane, a greatly-demanded raw material for nylon, a synthetic fiber, whereas diethylbenzene is hardly demanded and must be further converted into a useful compound, making diethylbenzene less favorable in an economic point of view.
Under these circumstances, the process (3) for converting ethylbenzene into benzene and ethane through dealkylation reaction has become predominant in recent years.
When ethylbenzene is effected dealkylation from a raw material of xylene isomer containing ethylbenzene to convert to benzene and then proceeding isomerization of ortho-xylene and meta-xylene to para-xylene, it is preferable to make the conversion of ethylbenzene as high as possible for the reduction of the cost of the separation of para-xylene. It is also preferable to reduce the loss of xylene as little as possible for reducing the original unit for the production of para-xylene and accordingly for lowering the cost of para-xylene production. From this point of view, an attempt of using zeolite, the crystallite size of which is greater than 1 micron (for example, Japanese Patent No. 1987-56138 (embodiments from 4 through 6 on page 10 and 11)), an attempt of reducing the diffusion speed of ortho-xylene (for example, Japanese Patent No.1996-16074 (the embodiment on page 5 through page 7)) and an attempt of using zeolite which has extremely high silica/alumina mole ratio of 500 or higher (for example, U.S. Pat. No. 4,163,028 (embodiments of 1 and 3 on page 14)) were done.
However, in these attempts, the loss of xylene is still high in relation to the conversion to ethylbenzene. Further, an attempt of decreasing the loss of xylene and increasing the conversion to ethylbenzene is not able to keep the isomerization of ortho-xylene and meta-xylene to para-xylene sufficiently high.